1. Field of the Invention
The present invention relates to permanent magnets, and in particular, to adjustable permanent magnet assembly for nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) applications.
2. Description of the Related Technology
During the last decade, a wide range of transportable, portable and single-sided NMR sensors and methodologies have been developed. The interest in such devices, as potential stand-alone or complementary alternatives to conventional MRI, arises from the obvious advantages of lower costs, portability, access to immovable arbitrary-sized objects, and scanning in the field. Mobile NMR measurements of, for example, relaxation times or MRI are conducted routinely in applications ranging from materials sciences to biomedicine. Recently, advances in methodology and hardware construction have allowed for high resolution NMR spectroscopy and medical imaging in one-sided, portable NMR/MRI systems.
Single-sided, portable systems feature strong magnets that produce either a remote homogeneous field or some type of natural gradient. The field homogeneity and size of the homogeneous region come at the expense of the size of these systems or the overall field strength. Magnets currently employed in portable NMR/MRI systems typically either have relatively small sensitive volumes, where the distance of this sensitive volume from the magnet often comes at the cost of field strength and hence signal, or have thin strips or slices of sensitive volume where the magnet's field usually has a strong gradient, which can cause a significant loss of signal with samples that have significant diffusion. With fixed magnets aiming for large sensitive volumes, as opposed to slices, one cannot tailor the field for specific applications, and, therefore, need to work with a fixed compromise between sensitive volume depth and field strength. For cases with fixed magnets one cannot adjust the field. With high gradient designs, the different volumes sampled by probe retuning are all high gradient and small volume. With a fixed sensitive volume magnet, one loses the low gradient large volume advantage if one doesn't work at that sensitive volume. Since field strength is sacrificed relative to a large gradient magnet to make a sensitive volume one, one has the disadvantages of both designs 1) low field, and 2) small volume large gradient when not working with the sensitive volume of the homogeneous volume type sensor.